Phase-inverting amplifying circuit



W. S. DRUZ PHASE-INVERTING AMPLIFYING CIRCUIT `Filed May 26, 1955 June 2, 1959 Patented June 2, 1959 ice 2,889,455 PHASE-mRTINGAMPLIFYmG CmcUrr Walter lS. Druz, Bensenville, Ill., assgnor to Zenith Radio Corporation, a corporation `of Delaware Application May 26, 1955, Serial N 0.511,225 2 lClaims. (Cl. Z50-27) This invention relates to a novel phase-inverting amplifying circuit for simultaneously translating two signals, occupying different portions of the frequentcy spectrum, with a different degree of amplification. The circuit is particularly attractive when incorporated in a subscription television receiver and for that reason is de'- scribed in such environment.`

In numerous subscription television systems the television signal is coded in accordance with a selected code schedule at the transmitter, and a coding signal representing or indicative of that `schedule is transmitted to subscriber receivers as a modulationrcomponent of the television signal itself. For example, in copending application Serial No. 326,107, filed December l5, 1952, and issued February ll, 1958 as Patent 2,823,252, in the name 'of .lack E. Bridges, and assigned to the presentfassignee, there is disclosed a system wherein the air-borne coding signal comprises combinations of signal bursts. The Ibursts individually have a predetermined identifying frequency and the combinations are transmitted to subscriber receivers along with the video signal, occurring during field-retrace intervals; The signal bursts are preferably randomly sequenced and randomly appearing within each combination and are derived from the video signal at the receiver by means of suitable filters. After separation from one another, they are available at the receiver for application to a transposition Yor switching mechanism just as they are applied to a like mechanism at the transmitter. This mechanism is employed in both locations to selectively establish particular circuit connections between input circuits, over which the bursts are conducted, and a plurality of output circuits which lead to a multi-stable actuating device such Vas a =bistable multivibrator.

Accordingly, the fbi-stable multivibrator is actuated in a prescribed sequence, shifting from one to the other of its stable-operating conditions as dictated by the combinations `ofi code signal bursts. The resulting amplitude changes in the output signal of the multivibrator are employed Vto effect mode changes in the television system by varying the relative timing of the video and synchronizing components. In this manner the operating mode is changed in Yan irregular or random fashion since the `individual bursts appear at random within the combinations of signal-bursts.

In addition to coding the telecast by altering the time 'relationship between the video and synchronizing i 2 verted, a phase inverting stage must be interposed in the video channel of a subscription television receiver. However, it should have unity gain for video; otherwise, the Ivideo signal would have `different amplitude levels for programs of inverted and conventional video polarity.

Usually, the received coding signal bursts are Aainplified to facilitate their use in decoding and it is highly desirable to 'be able to utilize a single amplifying `stage as a unity gain video phase inverter, when necessary, and at the same time as a high gain amplifier for the coding signal bursts. The novel circuit of the present invention is especially suited to serve such dual functions.

Briefly, in accordance with a preferred embodiment of the invention, the television signal which includes the videro and coding signals is applied to the control grid of a vacuum tube having a degenerative circuit coupled between its grid andV cathode. In addition, `a seriesresonant inductance coil-condenser combination is shunted across part of the degenerative circuit. Sincethe coding signal bursts occupy a relatively narrow portion of the frequency spectrum compared with the video signal, the resonant circuit may be tuned to the midepoint of this narrow portion effectively to short out the degenerative circuit for such signal bursts. In this way, degeneration is primarily effective for the video signal in order to achieve unity gain for video. On the other hand, the degenerative circuit is rendered ineffective for the coding signal 'bursts and the gain of the stage is considerable as to them. An equalizing network in the yform of `another series-resonant inductance coil-condenser combination shunts a portion of' the video load circuit and is tuned to the mid-point of the frequency range of the coding sig;- nal bursts to compensate the video signal for the effect of the frequency-selective shunt across the degenerative circuit. y

It is, accordingly, anobject of the present invention to provide a novel phase-inverting amplifying circuit [for simultaneously translating two signals, each having a substantially diflerent band width, with different degrees of amplification. t

It isV anotherA object of the linvention to provide a phase-inverting amplifying circuit for establishing `one amplification ratio for a first signal lying in a relatively narrow portion of the frequency spectrum and another, substantially uniform amplification ratio for a wide-band second signal overlapping the narrow band of the first signal.

It isa more specific object of the invention to provide a phase-inverting amplier for use in a subscription television receiver which phase inverts a video signal, if desired, with substantially no amplification while, during field-retrace intervals, amplies to a considerable degree an applied ceding signal.

The features of the invention which are believed to be new are set forth with particularity in the appended claims. The invention, together with further objects and advantages thereof, may 'best be understood, however, .by reference to the following description in conjunction with the accompanying drawing, the single figure of'which is aschematic diagram of a subscription television receiver including a phase-inverting amplifying circuit constructed in accordance with the invention.

The receiver, which may utilize a telecast originating at a transmitter constructed in accordance with the aforementioned disclosure by Bridges, comprises a radio-frequency amplifier 10 having input terminals connected to an antenna circuit 11 and output terminals connected to a first detector 12. This detector is coupled through an intermediate-frequency amplifier 13 to a seconddetector 14 which, iiiturn, is connected rto theinput `circuit cfa videoamplifier 15. 'Iihe output circuit of the video am-F pliier is coupled to a decoder 16 either directly or through a phase inverter 17, discussed in detail later, depending on the position of a manually-operated singlefpole double-throw toggle switch 18. The position in -which the' subscriber places this switch is determined by the video polarity of theparticular program to be viewed. The output terminals of decoder 16 are connected to the -input electrodes of a cathode-ray image-reproducing device 19.

, Decoder 16 may be similar to that disclosed and claimed in copending application Serial No. 243,039, led August 22, 1951, and issued August 7, 1956, as Patent 2,758,153, in the name of Robert-Adler, and assigned to the present assignee. It may comprise a beam-deflection tube having apair of output circuits which may-be selectively coupled into the video channel as the electron beam i: over that conductor, and to connect lthe third of the thereof is deflected from one fto the other of two seg- A mental anodes in synchronism with mode changes of the transmitted signal. It is assumed, of course, that these mode changes take the form of variations in the timing ofthe video signal relative to the synchronizing signal of the received composite television signal. Consequently, the output circuit coupled to one anode segment includes a time-delay network while the output connected to the other anode segment does not and the timing variations are compensated, effectively to decode the television signal, as the beam of the deflection tube is switched between its anodes. This switching effect is accomplished by means of a beam-deflection control or actuating signal applied to decoder 16, as explained hereinafter.

Second detector 14 is also coupled to a synchronizingsignal separator 22 which is coupled, in turn, to a eldsweep system 23 and to a line-sweep system 24. The output terminals of sweep systems 23 and 24 are connected respectively to fieldand line-deflection elements (not shown) associated with image reproducer 19.

In an illustrated embodiment of the Bridges system, i

the air-borne coding signal may comprise six bursts of various signal frequencies individually rtransmitted between .the line-drive pulses superimposed on the vertical blanking pulse after the post-equalizing pulses. To facilitate separating these signal bursts, it is desirable to provide circuitry which gates in only that portion of the composite television signal containing such bursts. To that end, field-drive pulses are derived from synchronizingsignal separator 22 and supplied-to a mono-stable multivibrator 25 having output terminals connected to one input Icircuit of a normally-closed gated amplifier 26. The output circuit of video ampliier 15 is also coupled through phase-inverting amplifying circuit 17 to another input circuit of gated amplifier 2.6 to supply the composite television signal thereto, and the output circuit of gate 26 is coupled to each one of a series off filter and rectifier circuits, schematically shown as one unit 27. Since the aforementioned Bridges application discloses a system wherein the coding information received during each fieldretrace interval includes signal bursts of six possible frequencies, unit 27 comprises six filter and rectifier circuits.

The output circuit of 'each of the lter-rectier networks is connected over a respective one of a series of conductors 31-36 to a transposition or switching mechanism 37. As farV as the technique of coding is concerned, the transposition mechanism is provided merely for the purpose of selectively connecting any one of conductors 31-36 to any one of three output conductors 41--43. Decoding can be accomplished only if the various ,interconnections established by switching mechanism 37 are identical to the corresponding interconnections established b y a similar transposition mechanism interposed between a source of burst signals and a bi-stable multivibrator in fthe coding ,apparatusVV at the transmitter.- The ,necessary information' for setting theftran'sposition mechanismis disseminated only to'authorized subscribers 4 v Y and a suitable charge may, of course, be assessed for such information.

Output conductors 41-43 from the transposition mechanism are connected respectively to different input circuits of a similar bi-stable multivibrator 44 comprising two cross-coupled triodes. One preferred arrangement is to connect one of conductors 41-43 to the control grid of one of the triodes so that multivibrator 44 will be actuated to` one of its two operating `conditions in response topulses applied over that conductor, ,to` connect another of the conductors to the control grid of Ythe other triode to actuate the multivibrator to the other of its operating conditions in response to'pulses applied conductors to the control grid of each triode so that the rnultibivratorV is actuated from. its instantaneous Condi- Y Y. 7

tion to its alternate condition in response to applied pulses over that third conductor. The output terminals of bi-stable multivibrator 44 are connected to ,the deflection-control elements of decoder 16 rto provide* an actuating or deection-control signal therefor which has` an amplitude excursion each time the multivibrator changes from one operating condition to the next.

In accordance with the present invention, phaseinverting amplier 17, which serves the dual function of inverting the phase of the video, when necessary, with unity gain and also of translating the air-borne coding signal bursts with considerable amplification, comprises an electron-discharge device 46 having an anode 47, cathode 48, screen grid 55, and control grid 49. The television signal is impressed on control grid 49 and it will be recalled that this signal, which is coded in accordance with a predetermined code schedule, includes video 'components occupying a relatively wide portion of the frequency spectrum and occurring during field-trace intervals. It further includes coding signal bursts representing the code schedule and occurring during intervening field-retrace intervals. The code signal bursts occupy a relatively narrow portion ofthe frequency spectrum which is completely over-lapped by Wide-band video components. The coding signal bursts are generally a downward modulation of the video carrier relativegf to black level and are within the same amplitude range of the carrier as the video components. A degenerative circuit is inserted between the control grid and cathode of tube 46 and includes a resistor 50 and a resistor 51 i connected in series between cathode 48 and a point of reference potential or ground. Control grid 49 is connected to the injunction between resistors 50 and 51 through a grid-leak return resistor 52. Resistor 51 is connected in the grid-cathode circuit of tube 46 via the ground connection and the output circuitry of video amplier 15 1 A frequency-selective :j circuit, consisting of an inductance coil 53 and a con-v denser 54 in series with a resistor 56, is bridged across f resistor 51 to shunt that part of the degenerative cathode in order to realize degeneration.

circuit.

The air-borne signalr bursts comprising the coding sig-V i nal are preferably within the 550 to 880 kilocycle range and the frequency-selective circuit is tuned to the mid frequency of that band. Consequently, it has an impedance within that relatively narrow portion of the spec- ,i trum which is extremely low compared to the resistance of the shunted part of the degenerative circuit. Resistor 56 preferably has a relatively small value in order to eect broad tuning over the entire range from 550 to 880 kilocycles. With this arrangement, the degenerative cirv Y,

cuit is predominantly eiective with respect to the video signal and is considerably less effective with respectto A. the coding signal bursts. In fact, if the circuit parameters areadjusted properly, pentode 46 maybe made to saturate, during the occurrence of the coding signal, in i' response to any unwanted extraneous signals having L amplitudes greater than thepositive peak of the coding signal, and may be madefto cut off in response to A.Anorle47 is coupled through series-connected resistorsl i S7, 5,8 and 59Hto the positive terminal of a source, of

, `,iiuidil'ectional potential 60, the negative terminal of which is ,connected to ground. Resistor 57 preferably `has a 'very high resistance compared Yto that ofresistors '58 composite video signal which Vincludes-the signal bursts 'constituting the coding-signal. The composite video sigand/or- 59 for reasons which will become apparent. Ari

...equalizing network, including an inductance'coil V61 in 'series` with a condenser 62, shunts resistor 59; ThisA net- Work is tunedto the same point as circuit 3, 54, 56 and l presents at the code signal burst frequencies an imped- `'ance which is low compared to the resistance ofkresistor 59; This equalizing network compensates the effect .of the frequency-selective circuit in theA cathode circuit .'t the translation of the video signal to achieve a substantially uniform frequency response to all the oom- -ponents of the video signal. Screen grid 55 is connected to the positive terminal of source 60 through a resistor 63v and is coupled to ground through a by-pass con- ANdenser 64. c The movable blade 66 of toggle switch 18 is connected to the input circuit of decoder 16 to selectively estab- A `lish a connection from either of two stationary contacts 67, 68. Contact 67 is connected to the junction of resistors 57 and 58 and contact 68 is connected to `grid 49.

` `t'actf67 Vis connected to the point along the load circuit `at which a signal of unity gainis derived, namely, a point `at which the video signal has substantially the same amplitude as, but inverted phase to, the input video signal. When no phase inversion is desired, switch blade 66 enc `gages contact 68 which removes the phase inverter from the video channel.

Inasmuch as a complete description of the coding techniqueis included in the copending Bridges application and since thecoding technique itself forms no part of the present invention,A the operation of the complete receiver will be described only briey, disregarding for the moment the detailed operation of phase-inverting amplifier 1,7. `The c :odedV television signal is intercepted by antenna 11, amplified in `radio-frequency Vamplifier 10, .heterodyned ,to the selected intermediate frequency in first fdetector 12, amp ed in nintermediate-frequency arnplier 13 and detecte-duin second detector 14 to produce i the coded composite video signal. This signal is amplified in video amplifier 15, translated to decoder 16 either directly or through phase inverter 17 depending on the position of switch 18, and impressed on the input electrodes of image-reproducing device 19 to control the intensity of the electron beam of the device in a wellknown manner.

The synchronizing components are separated in separator 22, the held-synchronizing pulses being utilized to synchronize the operation of sweep system 23 and consequently the vertical-deflection signal supplied to the field-deflection elements of reproducer 19, whereas the line-synchronizing pulses are utilized to synchronize sweep system 24 and therefore the horizontal-deiiection signal supplied to the horizontal-deflection elements in the image reproducer. Of course, the sound-modulated carrier wave normally received along with the video carrier is detected and reproduced in an appropriate audio system which has been omitted from the drawing for purposes of simplicity.

Field-drive pulses from separator 22 are supplied to mono-stable multivibrator 25 to produce a gating pulse for normally-closed gated amplifier 26. The parameters of the multivibrator are so chosen as to overlap, in point of time, that portion of the field-retrace interval of the nal from amplifier-'15 is continuously applied, through phase-inverter amplifierV 17, tothe input circuit of'lamplifier 26, but only the information contained during the interval of the gating pulse is translated to filter and rectifier unit 27. Amplifier 26 is thus open during the times the signal bursts of various frequencies, Arepresenting the coding schedule of the program, are received and since the filter and rectifier circuits are individually tuned to an assigned one of these frequencies, such bursts are separated out from the composite video signal and from each other. Each time a burst of signal frequency occurs in the coding signal, it is channeled over a corresponding input -circuit 31-36 through transposition mechanism 37 to a selected one of the input circuits 41-43 of bi-stable multivibrator 44. The coding app-aratus at the transmitter comprises an identical arrangemeut of a bi-stable multivibrator connecter to a source of air code signal bursts through a transposition mechanism so if the transposition mechanism of the receiver is `adjusted to the same setting as the transportation mechanism at the transmitter, the input circuits of bistable actuating device 44 receive pulses similar to those received by the input circuits of the corresponding bistable actuating device at the transmitter. Multivibrator 44 therefore produces a rectangularly-shaped actuating or deflection-control signal for decoder 16 which is identical in Wave form but opposite in phase t-o that developed at the transmitter for coding the television signal initially. It undergoes an amplitude excursion each time thezmultivibrator is triggered from one of its operating conditions to the other. Decoder 16 consequently operates in synchronism with the coder at the transmitter so that the signal applied to the input circuit of image reproducing device 19 is suitably compensated to effect intelligible p reproduction.

Vand 51.

Unity gain Yis not desired, however, for the air-borne coding signal bursts since maximum amplitude and preferably limitingof those Abursts is required. Accordingly, series-resonant circuit 53, 54 is tuned to short out degenerative resistor 51 at the frequencies of these bursts. With decreased degeneration, the amplification or gain is increased and limiting of the air code signal bursts occurs in tube 46 under all lbut very weak signal conditions. The coding signal is derived from a different portion of the anode load circuit than the video signal; specically, the load circuit for the coding signal includes resistor 57 which is of relatively high value.

In view of the fact that the video signal Ihas a frequency range from 0 to 4.5 megacycles, some components necessarily fall within the narrow range for which the amplifier has increased gain. Unless compensated, this would result in a non-uniform frequency response characteristic, and, consequently the equalizing network 61, 62 shunts part of the video load circuit, namely resistor 59. Thus, a substantially uniform frequency response is realized in respect of the video signal.

While the invention is not limited to any specific constants or components, the following electrical dimensions of the embodiment shown are illustrative of a proven phase-inverting amplifying circuit:

The invention therefore provides a phase-inverting amplifying circuit which amplifes to a considerable extent the air-borne coding signal bursts, having components occupying a relatively narrow portion of the frequency spectrum, and amplifies to a' negligible extent the video signal, which has components occupying a relatively wide portion of the yfrequency spectrum encompassing the aforesaid narrow portion, while at the same time providing uniform frequency response for the video signal.

Certain features described in the present applicationi are disclosed and claimed in copending application Serial No. 700,855, led December 5, 1957, in thetname of Jacob M. Sacks, also assigned to the present'assignee.

While a particular embodiment of the invention has been shown and described, modifications may be made;

occupying a relatively wide frequency range overlappin'g at' least part of said narrow range, and further including frequency-selective degenerative means coupled between said control grid and said cathode and predominantly .effective with respect to the components of said wide band ,signalv exhibiting frequencies outside of saidnarrow fre`v quency range to provide higher gain for said narrow .band signal thanfor'said wide band signal, the improvef ment comprisingra load circuit including a plurality of series-connected impedance elements coupled to said anode; means forderivingsaid wide band signal from one portion of said load circuit including a predetermined pair'of-said impedance elements; means for deriving said narrow band signal from a different portion of said load Resistor 52 megnhms 1 Ylectively reducing the effective impedance of only one Resistor 56 ohms 82 of said impedance elements to the components of said Resistor -57 y do 4.7K wide band signal having frequencies within said narrow Resistor `58 do 150 frequency range relative to-its impedance' tofthfefren'ain- Resistor 59 do v 390 5 ing components ofsaid wide band signaltoicompenste Resistor 63 do 150K the effectofV saidfrequency-selective `deg`enerative`-means Condenser 54 micro-micro-farads" 820. 'on the vtranslationof the component-sof ,saidfwide band Condenser -62 do 820 n signal exhibiting frequencies within-said narrowrange, Condenser 64 micro-farads v A 20y to provide. a substantially uniform frequencyresponse Coil 53 micro-henries 58 10 to all of the components of said wide band signaL v Coil `61 do 58 2. YIn a phase-inverting amplifying circuit including-Yan 4Potential of source 60 volts 250 electron-discharge device having an anode, a 'cathode and a control grid upon which grid is impressed a composite signal containing a narrow band signal having components occupying a relatively narrow frequency range'and a wide-band signal having components occupying arelatively'wide frequency range overlapping at least 'partof `said narrow range, and further including afreque'ncyselective degenerative means coupled between said control grid, and said cathode' and also to `a point'of reference potential and predominantly'elfective with respect to the components of said wideband signalexhibiting frequencies outside of said narrow'frequen'cyrange to provide higher gain for said narrow band signalnthan for said wide band signal, the improvement comprising: a loadfcircuit including a; load vresistorvcoupled:between said anoderandsaid point of reference potential; means 'for deriving"l said wide band signal from a take-off ,point along said resistor; means for deriving said narrow band signal from said anode; and an. equaliz'ing network, 'including ajseiies-resonant inductance coil andcondenser Y combination .tuned to a frequency within Ysaid narrow Arange, shunting at least part of said load resistor between .said take-off point :and `said pointof reference potential and having an impedance, within said narrow frequency range, 'which is'low compared to the resistance ofthe shunted'part of said loadrresistor 'to compensatevthe effect of said frequency-selective degenerative means on the translation of the components ofsaid wideband signal, within saidrnarrow range for achieving a substantially uniform frequency responseto all of thlegcc'ginpo 1 nentsofsaid wide band signal. t

circuit; andfrequency-'selective equalizing means'for se- References Cited the le of4 this patent l UNITED STATES PATENTs-y 2,293,262' Koch Y Aug'.'18, 1942 2,351,934 u De Kramolin June 20,'-51944 u 2,724,050 Y Bos'cia .Nov.'15,`.1955 2,735,002 Keizer et al. Feb'. 14, V:1956

Wilson Sept.` 16,11'941 Denton June 25, 1957 

